St. Hermenegild, a Spanish prince, was a convert from Arianism, thanks to the influence of his Catholic wife and the instructions and prayers of St. Leander. His conversion so angered ;his father, the king, that the monarch had him cast into prison.
On Easter night a heretical Arian bishop was sent to him with Holy Communion, but he refused to receive even the Easter Communion from the hands of a heretic, and his father then put him to death. St. Gregory the Great learned of his martyrdom while at Constantinople and inserted the account in the third book of his Dialogues.
(Source: Fr. Lasance, The New Roman Missal)